The issue with the extra spike at the beginning of the colored noise injection was related to time settings in the ACL commands in the ENVnoise process. These modifications were made by Alain (on March 26):

Figure 1 shows the result of the test: blue is with the OLD settings, purple is the with the NEW settings (the same colored noise with amplitude 0.025 V has been injected in both cases). The spike is no longer present.

The same test was done by injecting the colored noise from the ENVnoise and also using the ENV_MAIN node which calls the AcousticInjectionCEB process.

On April 9 we performed a set of short (60s) colored acoustic injections in the CEB. The ITF was locked in LOWNOISE 3 since a few minutes.

The purpose was to tune the level of the injection in such a way to avoid triggering the 92 Hz oscillation of the BS F0 accelerometers (mostly Sa_BS_F0_ACC_H3) which caused unlocks in the past (https://logbook.virgo-gw.eu/virgo/?r=63709). The purpose of the present test was not to understand the nature and physical reason of the oscillation onset, which remains to be understood.

We injected colored noise between 20 and 2000Hz with 3 levels: 0.03 V, 0.04 V and 0.05 V, one 25min glitch occurred right at the beginning of the 0.05 V injection (Figure 1).

The intensity on the acoustic noise produced in the hall, measured in the position of the hall microphone, was of the order of a factor 20 above the quiet condition (Figure 2). The 92 Hz oscillation of  the BS F0 ACC H3 accelerometer onsets already with the 0.03 V but  amplifies x5 and x10 when the injected level increases from 0.03V to 0.04V and from 0.03V to 0.05V, respectively (Figure 3).

Figure 4 shows the impact on hrec. The noise in hrec increased at some structures, mostly known to be associated to SDB1. These excitations are well visible with all injected levels (Figure 5, which also shows the 25min glitch).

It seems to notice also an increase of the 74 Hz line (DARM loop gain) and the 227 Hz line (SSFS loop gain). As well, it seems to notice a small increase of the noise in the bucket. It seems that particularly the injection at 0.05 V, brings the ITF to a different working point where other noises (e.g. frequency noise) are more coupled.

However, spectrograms in Figures 6 and 7 indicate that the amplitude of 74 Hz and 227 Hz lines was not so stable during the injection period (we were at the beginning of a lock) and its correlation with single acoustic injections is not straightforward.

Paolo's suggestion is to better study this behaviour with longer injections.

We performed a set of four acoustic injections in CEB with the AcousticInjectionCEB VPM process:

1) 17:07:55 UTC - (8 - 2000) Hz, 600 sec, 0.03 V

2) 17:21:05 UTC - (8 - 2000) Hz, 600 sec, 0.04 V  (25 min glitch at ~17:29:24 UTC)

3) 17:33:48 UTC - (8 - 2000 )Hz, 600 sec, 0.05 V

4) 18:04:40 UTC - (80 -100) Hz, 300 sec, 0.04 V

The directory path of output files is /virgoData/NoiseInjections/AcousticInjectionsO4/output. The list of file injections is reported below:

1) AcousticColored_NOISE_CEB_DetTerrace-1397495261.txt

2) AcousticColored_NOISE_CEB_DetTerrace-1397496052.txt

3) AcousticColored_NOISE_CEB_DetTerrace-1397496814.txt

4) AcousticColored_NOISE_CEB_DetTerrace-1397498666.txt

We tested the sensitivity of the top-stage BS accelerometers at three different levels of acoustic injection (nominally 0.03 - 0.04 - 0.05 V).

• Pink = no noise, Orange = 0,03, Green = 0.04, Grey = 0.05, Blue = narrow filter

Noise was filtered from 8 to 2000 Hz, but the actual noise produced was only from 20 Hz and up, as the speaker is unable to go lower.

The fourth injection (18:04:40 UTC - (80 -100) Hz, 300 sec, 0.04 V) aimed to produce the same level around the frequency of ~93 Hz (without generating out-of-band noise) where one of the horizontal BS F0 accelerometers (Sa_BS_F0_ACC_H3_500Hz) is usually excited during acoustic injections, in order to check whether the coupling is linear or not.

• Blue = no noise, Green = narrow noise, Pink = broadband noise

Unfortunately, the filter was too narrow, and the noise spectral density at 93 Hz was lower than expected.

However, we can say that generating a higher level by a factor of 2 (with no noise at lower frequencies) resulted in an increase of the peak by about a factor of 2 (difficult to estimate accurately). It appears that there is no upconversion and the system is "linear."

During this test we also noticed that there is some (unexplained) coherence between the upper stage F0 accelerometers and the lower external magnetometers (this is also the case in other towers besides BS).

At first glance one might think of general coupling due to the external accelerometer cables running not too far from the magnetometers (as indeed they are), but there are high-level signals in the acceleration spectrum that have no coherence. If it were a coupling from the cable, it should be broadband.

This is a first (preliminary) look at the effect of acoustic injection into Hrec, performed on Thursday 18.

For each injection (at the 0.03, 0.04 and 0.05 V levels) the BNS range decreased.

Here the entire Hrec range from 10 to 1000 Hz.

There are regions growing at low frequency (around 120 and 170 Hz) and high frequency (the most relevant from 500 to 540 Hz).

Below 100 Hz nothing seems to have changed significantly.